Recently, an increasing desire for better fuel economy of vehicles demands tire rubber compositions excellent in fuel economy. Reducing the hysteresis loss of a rubber composition is generally effective in improving the fuel economy.
Tire rubber compositions generally contain carbon black as a filler because it provides a good reinforcing effect and favorable abrasion resistance. In order to improve the fuel economy in a carbon black formulation, using carbon black having a large particle size or reducing the amount of carbon black may be considered. In such cases, however, the rubber strength, abrasion resistance, wet grip performance, and the like are reduced. Thus, there still remains room for improvement.
The use of silica instead of carbon black is known to improve the fuel economy. However, since silica provides lower reinforcement than carbon black, the rubber strength and the like are reduced in such a case. Thus, there still remains room for improvement.
As the method for improving the fuel economy, Patent Literature 1 discloses the addition of a specific polar group into a rubber in a silica formulation to enhance the dispersibility of silica. In recent years, however, further improvement of the fuel economy has been demanded. In addition, the method still leaves room for improvement in that the rubber strength is lower compared with carbon black formulations.
As another method for improving the fuel economy, Patent Literature 2 discloses the addition of an amine compound to enhance the dispersibility of carbon black. This method still leaves room for improvement in improving the fuel economy, rubber strength, abrasion resistance, and processability in a balanced manner.
Another known method for improving the fuel economy is to form a tread portion having a two-layer structure including a base tread and a cap tread and use a rubber composition excellent in fuel economy in the base tread. Patent Literature 3 discloses a method for improving the fuel economy and handling stability by using a tin-modified butadiene rubber and paper fibers. The method still leaves room for improvement in improving the fuel economy, handling stability, and processability in a balanced manner.
Meanwhile, recently, rubber compositions for fiber cord toppings are increasingly required to provide improvement in fuel economy, handling stability including braking performance during running at high speeds, and riding comfort. A known method for improving the handling stability is to add a phenolic resin to a rubber composition for a fiber cord topping to improve the complex modulus of elasticity (E*), thereby enhancing the rigidity. This method, however, increases tan δ and deteriorates the fuel economy. Thus, there still remains room for improvement.
A known method for improving E* and reducing tan δ is to crosslink a cross-linkable resin such as a resorcinol condensate or modified resorcinol condensate with a methylene donor such as a partial condensate of hexamethylol melamine pentamethyl ether (HMMPME) or a partial condensate of hexamethoxymethylolmelamine (HMMM). Patent Literature 4 discloses a rubber composition having better adhesion to carcass cords and suppressing crack growth, heat build-up and a reduction in rubber properties. In recent years, however, further improvement of the fuel economy has been demanded.